In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot combustion gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the respective turbine blades and vanes, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion and exhaust gas temperatures. The maximum temperature of the combustion gases is normally limited by the materials used to fabricate the hot-section components of the engine. These components include the turbine vanes and turbine blades of the gas turbine, upon which the hot combustion gases directly impinge. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of up to about 1800–2100° F. At these temperatures, the components are subject to damage by oxidation and corrosion.
In one approach used to protect the hot-section components against oxidation and corrosion, a portion of the surface of the turbine blades is coated with a protective coating. One type of protective coating is an aluminum-containing protective coating deposited upon the substrate material to be protected. The deposited aluminum-containing coating interdiffuses into the substrate material, and the exposed surface of the aluminum-containing protective coating oxidizes to produce an adherent aluminum oxide scale that protects the underlying substrate.
Several techniques are available to coat the exterior surfaces of the turbine blades and vanes. However, in some cases the airfoil sections are hollow, to permit a through-flow of cooling air or to reduce the weight of the airfoil section, or for both reasons. The uniform coating of the exterior and interior surfaces of the hollow sections is difficult to achieve, particularly where there is no possibility for an end-to-end flow through of a coating vapor and particularly in refurbishment operations after the turbine blades or vanes have been used in service and are returned for refurbishment.
There is a need for an approach to coating the exterior surfaces and also the interior surfaces of such components, with an aluminide coating that is reasonably uniform in thickness. The present invention fulfills this need, and further provides related advantages.